wake -
The region of turbulence immediately to the rear of a solid body in motion relative to a fluid. Under certain conditions a series of vortices may form in the wake and extend downstream; such a vortex train in a turbulent wake is called a vortex street. Compare lee eddies.

warm air mass -
See airmass classification.

warning -
Issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very highprobability of occurring. A warning is used for conditions posing a threat to life or property. Compare watch, weather advisory.

watch -
Issued when the risk of a hazardous weather or hydrologic event has increased significantly but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough leadtime so that those who need to set their plans in motion can do so. Compare warning, weather advisory.

water -
1. A transparent, colorless, odorless, and tasteless liquid found near the surface of the earth. In the lithosphere, hydrosphere, and atmosphere of the earth, water is found as a gas, liquid, and solid. Water falls from the clouds as rain, hail, sleet, graupel, snow, etc., and runs off and through soils to form creeks, streams, rivers, and lakes. In its solid form, it is referred to as ice or snow. Water as a liquid and as ice covers 70.8% of the surface of the earth and plays a fundamental part in the earth-atmosphere energy balance. Water (chemical formula H2O) corresponds to two parts hydrogen and one part oxygen on a molecular basis; by weight, water is 11.19% hydrogen and 88.81% oxygen. Water has a melting point of 0°C (32°F), a boiling point of 100°C (212°F), and a specificgravity of 1.000 at 4°C (39°F), by definition. 2. Can refer to a body of water, such as a lake or a stream, or even a larger body of water such as a sea or part of an ocean, for example, international waters. 3. Used to describe water in specific locales; for example, hydrologists refer to soil water, surface water, and groundwater. 4. As a verb, used to describe irrigation corresponding to the application of water to plants, the grounds surrounding a residence, or to a garden.

water vapor -
(Also called aqueous vapor, moisture.) Water substance in vapor form; one of the most important of all constituents of the atmosphere. Its amount varies widely in space and time due to the great variety of both "sources" of evaporation and "sinks" of condensation that provide active motivation to the hydrologic cycle. Approximately half of all of the atmospheric water vapor is found below 2-km altitude, and only a minute fraction of the total occurs above the tropopause. Water vapor is important not only as the raw material for cloud and rain and snow, but also as a vehicle for the transport of energy (latent heat) and as a regulator of planetary temperatures through absorption and emission of radiation, most significantly in the thermal infrared (the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as absolute humidity, mixing ratio, dewpoint, relative humidity, specific humidity, and vapor pressure.

water vapor imagery -
A display of data taken in one of the water vapor channels, for example, 6.7 or 7.3 m[&mgr;]m. Atmospheric water vapor absorbs outgoing radiation in these regions, resulting in a decreased temperature being sensed by the satellite.

wave -
1. Generally, any pattern with some roughly identifiable periodicity in time and/or space. This applies, in meteorology, to atmospheric waves in the horizontal flow pattern (e.g., Rossby wave, long wave, short wave, cyclone wave, barotropic disturbance). See also inertia wave. 2. At the surface of the ocean, a disturbance generated by wind action with dynamics governed by the influence of gravity and/or surface tension. See ocean waves. 3. Popularly used as a synonym for "surge" or "influx," as in tidal wave (storm surge), heat wave, cold wave.

wavelength -
In radiation, the distance between periodic spatial repetitions of an electromagnetic wave at a given instant of time; used extensively to classify the nature of the radiation, since most of the interactions between radiation and matter are extremely sensitive to the wavelength of the radiation. Units are length (e.g., nm, m[&mgr;]m, mm, cm, with conventional usage depending on which part of the electromagnetic spectrum is being considered).

weather -
The state of the atmosphere, mainly with respect to its effects upon life and human activities. As distinguished from climate, weather consists of the short-term (minutes to days) variations in the atmosphere. Popularly, weather is thought of in terms of temperature, humidity, precipitation, cloudiness, visibility, and wind. 2. As used in the taking of surface weather observations, a category of individual and combined atmospheric phenomena that must be drawn upon to describe the local atmospheric activity at the time of observation. Listed weather types include tornado, waterspout, funnel cloud, thunderstorm and severe storm, liquid precipitation (drizzle, rain, rain showers), freezingprecipitation (freezing drizzle, freezing rain), and frozen precipitation (snow, snow pellets, snow grains, hail, ice pellets, ice crystals). These elements, with the exception of the first three, are denoted by a letter code in the observation. With the METAR code, reporting weather also includes an intensity qualifier (light, moderate, or heavy) or proximity qualifier. The weather used in synopticweather observations and marine weather observations is reported in two categories, "present weather" and "past weather." The "present weather" table consists of 100 possible conditions, with 10 possibilities for "past weather"; both are encoded numerically. Another method, which has the advantage of being independent of language, is the recording of weather types using symbols. There are 100 symbols that identify with the numeric codes of the synoptic observation. 3. To undergo change due to exposure to the atmosphere. See also weathering.

weather observation -
In general, an evaluation of one or more meteorological elements that describe the state of the atmosphere either at the earth's surface or aloft. Surface weather observations and upper-air observations are the major categories, with a number of subtypes, but separate from these are such categories as radar meteorological observations, sferics, and solar-radiation observations.

WER -
Abbreviation for weak echo region.

westerlies -
1. (Also called circumpolar westerlies, circumpolar whirl, countertrades, middle-latitude westerlies, polar westerlies, subpolar westerlies, subtropical westerlies, temperate westerlies, zonal westerlies, westerly belt, zonal winds.) Specifically, the dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres. At the earth's surface, the westerly belt (or west-wind belt, etc.) extends, on the average, from about 35° to 65° latitude. At upper levels, the westerlies extend farther equatorward and poleward. The equatorward boundary is fairly well defined by the subtropical highpressure belt; the poleward boundary is quite diffuse and variable. Especially in the Northern Hemisphere, even the annual averagewesterlies are markedly enhanced in some regions, namely, the jet streams. See polar vortex, antitrades, tropical easterlies, zonal index. 2. Generally, any winds with components from the west. 3. See equatorial westerlies.

wind -
Air in motion relative to the surface of the earth. Since vertical components of atmospheric motion are relatively small, especially near the surface of the earth, meteorologists use the term to denote almost exclusively the horizontal component. Vertical winds are usually identified as such. Surface winds are measured by anemometer and wind vane; winds aloft by such systems as pilot balloon, radiosonde, or aircraft navigational techniques. See also circulation, general circulation, turbulence, geostrophic wind, gradient wind, local winds, Beaufort wind scale, draft, cyclone, whirlwind, squall, storm.

wind direction -
The direction from which the wind is blowing. See also prevailing wind direction.

wind speed -
Ratio of the distance covered by the air to the time taken to cover it. The instantaneous speed corresponds to the case of an infinitely small time interval. The mean speed corresponds to the case of a finite time interval. It is one component of wind velocity, the other being wind direction).

winds aloft -
(Also called upper winds, upper-level winds.) Generally, the wind speeds and directions at various levels in the atmosphere above the domain of surface weather observations, as determined by any of the methods of winds-aloft observation.

windward -
The upwind direction from a point, for example, westward in the case of a west wind.

windward side -
The side of a mountain, ridge, or other flow obstacle facing toward the direction of the large-scale or ridge-top wind; the upwind side; opposite of leeward.

winter -
Astronomically, between the winter solstice and vernal equinox in the Northern Hemisphere, and the summer solstice and autumnal equinox in the Southern Hemisphere; the coldest season of the year; the "low sun" season during which the sun is over the opposite hemisphere; the "hibernal" season. Popularly and for most meteorological purposes, winter is taken to include December, January, and February in the Northern Hemisphere, and in the Southern Hemisphere, June, July, and August; the opposite of summer. 2. See Blackthorn winter.

work -
A form of energy arising from the motion of a system against a force, existing only in the process of energy conversion. Many forms of work (electrical, chemical, etc.) may be defined by analogy with mechanical work, but in meteorology the most frequently useful mathematical expression is that for the work per unit mass w done by a gaseous system in a given reversible process from thermodynamic state s1 to state s2: w = pda[&agr;], where p is the pressure of the system and a[&agr;] its specific volume. The amount of work done will be a function of the particular process as well as of the initial and final states. See heat, first law of thermodynamics.